Implement walkthrough state provider

[nzaytsev]

Description

• Map usage tracker data to context to automatically set walkthrough checkboxes
• Adds progress line and badge to home view

The feature is disabled. To enable use setting flag gitlens.test.newWalkthrough

Checklist

• [x] I have followed the guidelines in the Contributing document
• [x] My changes follow the coding style of this project
• [x] My changes build without any errors or warnings
• [x] My changes have been formatted and linted
• [ ] My changes include any required corresponding changes to the documentation (including CHANGELOG.md and README.md)
• [x] My changes have been rebased and squashed to the minimal number (typically 1) of relevant commits
• [x] My changes have a descriptive commit message with a short title, including a Fixes $XXX - or Closes #XXX - prefix to auto-close the issue that your PR addresses